i) Field of the Invention
This invention relates to a biofilter for purifying waste waters and/or waste gases, to a method of purification of waste waters and/or waste gases using same, and to a composition for the purification of waste waters and/or waste gases. The invention more specifically relates to a biofilter for the simultaneous purification of domestic, urban, industrial, or agricultural waste waters and waste gases via an organic fixed bed support which can be composted.
ii) Description of Prior Art
The potential of biofiltration for the purification of waste waters and waste gases has been recognized due to its efficacy and low cost. The organic support acts mainly at two levels: a) it can directly retain or fix different types of pollutants, and b) it can act as a support for various types of microorganisms capable of degrading the retained substances. The constituents of organic supports, lignin and organic acids in particular possess numerous functional polar groups conferring thereto a good absorption capacity for organic molecules as well as transition metals (Coupal et al., 1976, Wat. Res. 10: 1071-1076).
Although a number of treatment systems to control the discharge of pollutants have been disclosed, due to economic, climatic, or functional reasons and also due to their inherent complexities, they have unfortunately proven to lack versatility or practicality in a number of situations. Thus, versatile treatment systems that can be efficacious for the purification of waste waters having different particular characteristics (urban vs agricultural waste waters for example) have yet to be designed.
One of the organic support of choice is peat because it is a polar and highly porous material, in addition to being inexpensive and easy to use. Under natural conditions, peat adsorbs many undesirable elements and reduces their level from the environment (McLelland et al., 1986, Internat. Peat J. 1: 1-14; Couillard, 1994, Wat. Res., 28: 1261-1274). In recent years there has been an increasing interest in the incorporation of peat into waste water or waste gas treatment systems. Peat removes pollutants and microbial activity from waste water by adsorption and absorption reactions. Peat has also been shown to be effective at removing suspended solids, nutrients, heavy metals, organic matter, oils, odors and bacteria (Couillard, 1994, Wat. Res., 28: 1261-1274; the contents of which are incorporated herein by reference).
Several designs for peat filter beds to treat domestic waste water effluents have been developed and their construction is essentially similar (Couillard, 1994, Wat. Res., 28: 1261-1274). They are usually composed of four layers: (from the bottom up) a layer of coarse gravel, a layer of sand, a layer of peat and finally, a layer of grass. The two bottom layers act as a support to retain the peat bed. However, these biofilters are reported as being limited to the treatment of small volumes of waste water disposal (Couillard, 1994, Wat. Res., 28: 1261-1274).
Biofilters for the purification of waste gases have been disclosed for example in U.S. Pat. No. 4,662,900, to Ottengraf et al., which discloses a method of biologically filtering waste gases, by passing them through a layer of carrier material provided with an appropriate microorganism. The carrier material comprises material such as peat and wood chips. The biofilter described therein is a multi-chambered filter which comprises a pretreatment chamber that permits a spraying of the gases prior to their passage through the carrier material. Further U.S. Pat. No. 4,662,900 teaches that spraying water onto or into the carrier material is not sufficient to permit an adequate humidity control thereof. The incorporation of additional materials to the carrier material to prevent the aging thereof is also disclosed. These materials are mainly inert but can in addition comprise organic materials such as particles of car tires, ground lava bits and other non-composting materials.
Methods for the purification of waste waters and filters therefor have also been disclosed, for example in U.S. Pat. Nos. 4,461,708 to Hakulinen et al., and 4,561,974 to Bernard et al., U.S. Pat. No. 4,461,708 discloses a fluidized bed reactor, particularly suited to the wood-processing industry. The preferred procedure involves anaerobic and aerobic steps. The aerobic step involves a biofilter having a filtering material comprised in part of pieces of plastic. To produce aerobic conditions, air is conducted in the lower part of the biofilter, in close proximity to the water outlet. U.S. Pat. No. 4,561,974 on the other hand discloses an apparatus for the anaerobic filtration of waste water, with the filtering material comprising a plurality of vertically extending tubes or plates.
One apparatus and method for the simultaneous purification of air and water has also been disclosed in U.S. Pat. No. 5,080,793 to Urlings. In this patent, a complex filtering device is disclosed, in which air and water move upwardly in the first filtering unit. The carrier material providing support for microorganisms comprises small blocks and/or solid plates of porous polyurethane.
Although an organic support such as peat is an ideal biological filter material for water or gas waste treatment systems, this organic support often gets compacted, thereby significantly reducing the flow rate of the gas or water to be filtered. Methods of pre-treating peat to diminish the clogging problems of biofilters utilizing a peat mixture have been disclosed (Couillard, 1994, Wat. Res., 28: 1261-1274). Nevertheless, the compaction and aging (shrinking, cracking, drying, and development of anaerobic zones) of the peat mixture is not significantly prevented. A continuous movement of the organic support to prevent this aging phenomenon has also been disclosed (U.S. Pat. No. 4,662,900 and the references cited therein), but such a mixing is generally impractical.
Although various types of biofilters have been. designed in order to purify waste waters or waste gases there is still a need for a polyvalent biofilter that could simultaneously purify waste waters and waste gases. Further, the availability of a biofilter comprising a filtering material for the purification of waste waters and/or waste gases would be of advantage for the water treatment and gas treatment industries.